If you wish to merge your all css file in Magento you can do that by your Magento Admin Panel. Log in to your Magento admin panel then go to System->Configuration. From the Current Configuration Scope: Select your Store view then click on Developer tab.Now click on Css Settings Tab .From the Merge CSS Files Drop down select yes to merge your css. Default No is selected . Now clear your browser cache and also refresh your Magento cache.You all css file is merged Now.